In the transmission of electrical signals within an assembly, or between assemblies, it can be required to transmit both differential signals and also non-differential signals, so-called single-ended signals, via transmission lines. This can for example occur if signals have to be transmitted from a plurality of different components, or to a plurality of different components, that provide different types of transmission of a particular signal, differential or single-ended, so that their transceiver circuits have to be matched to one another with regard to a differential and a single-ended signal transmission.
The components are therefore provided with transceiver circuits that have a differential amplifier and that have at least one single-ended transmit amplifier, or a difference amplifier, and at least one receive amplifier.
A transmission of single-ended signals via a transmission line pair from a transmit circuit to a receive circuit, of which at least one has a differential amplifier or a difference amplifier, results in an increased energy consumption. This is because the impedance matching resistance assigned to the differential amplifier of the transmit circuit, required in the case of differential signals, and the terminating resistance assigned to the difference amplifier of the receive circuit causes, in the respective circuit part, a current flow between the two single-ended signals if the signal levels of the single-ended signals on the transmission lines assume different signal levels. The problem is exacerbated in that the voltage levels used for single-ended signals are significantly higher than the voltage levels of differential signal parts. The voltage levels of single-ended signals can be 5 volts, so that for example given an impedance matching resistance/terminating resistance of 100 ohms, a flow of current of 50 mA can result. This relatively high current flow can impair the output signals of the amplifiers of the single-ended signals, or can even exceed their capacity. In addition, this current flow can cause crosstalk between the two single-ended signals, so that the single-ended signals may possibly no longer be capable of being unambiguously recognized by the respective receive circuit. Therefore, it is standard to separate the differential circuit parts using a switch if the transmission lines are used for the transmission of single-ended signals.
Because as a rule the transmission of differential signals takes place at high frequencies, the provision of additional switches for separating the transmit part or receive part for the differential signal in the transmit circuit or in the receive circuit is disadvantageous due to the parasitic capacitances and reflections thereby introduced to the required line branchings, because they can impair the quality of the transmission of the differential signal.
Various approaches are known from the existing art for the separation of the differential circuit parts using one or more switches. These approaches provide either separating only the impedance matching resistance or the terminating resistance for the differential signal or for the overall differential circuit part. In this way, a loss of power at the impedance matching resistance or terminating resistance can be avoided.
The introduction of additional switches and/or line branches can make it more difficult to meet the high demands made on the transmission lines for the transmission of differential signals.
In addition, the provision of only one switch for separating at least the impedance matching resistance/terminating resistance causes, in some approaches, an asymmetrical capacitive load of a transmitted differential signal.